A Phase 1 Study of HLD-0117 in Patients With Estrogen Receptor Positive (ER+) Metastatic Breast Cancer (MBC)

Inclusion Criteria: * Female (assigned at birth), ≥18 years old, and able to provide informed consent * Histologically confirmed metastatic or locally advanced breast cancer * Postmenopausal status defined by surgical or natural menopause, or ovarian suppression with a GnRH agonist * Prior treatment including at least one endocrine therapy in the metastatic setting, at least one CDK4/6 inhibitor (in the adjuvant and/or metastatic setting), and no more than two prior cytotoxic regimens in the metastatic setting * Radiologic disease progression on the most recent therapy * Measurable disease per RECIST v1.1 * Willingness to provide baseline and on-treatment tumor biopsies, unless not feasible or medically appropriate * ER-positive and HER2-negative status documented within 2 years * ECOG performance status 0-1 and life expectancy of at least 3 months * Adequate organ function Recovery from prior therapy-related toxicities to Grade ≤1 (except alopecia; neuropathy and endocrinopathies ≤Grade 2) * Ability to swallow oral medication and comply with study procedures * Stable dose (≥30 days) of bisphosphonates or denosumab, if applicable Exclusion Criteria: * Inflammatory breast cancer or known brain metastases * Recent major bleeding or uncontrolled bleeding disorder * Ongoing corticosteroid use \>10 mg/day (prednisone equivalent) * Recent anticancer or investigational therapy within 14 days (28 days for fulvestrant) * Untreated or unstable spinal cord compression * Significant cardiovascular disease within 6 months or ongoing uncontrolled cardiac conditions * Active or uncontrolled infection (controlled HIV or treated hepatitis C allowed) * Uncontrolled renal, pancreatic, or liver disease (excluding stable conditions such as Gilbert's syndrome or liver metastases) * Another malignancy requiring treatment within 2 years (except low-risk, curatively treated cancers) * Major surgery within 28 days * Any condition that may interfere with safety or study compliance * Pregnancy or breastfeeding